Methods for manufacturing relatively coarse granular urea once adopted the one using a drum and the one using a pan granulator. Those methods, however, have drawbacks that once-granulated urea has to be recycled to the granulator after cooling the granular urea in order to release the urea-solidification heat generated during granulation to operate the process under a suitable condition of internal temperature of the granulator for the granulation, that a large apparatus is necessary for establishing the recycling, that there is a limitation of production capacity per single unit of granulator, which requires a plurality of granulators at recent industrial scale, and that the urea being sprayed into the granulator is a molten urea having equal water content to that of the product urea.
As a result, recently, the granulation method is principally fluid bed one or spout-fluid bed one. As of these granulation methods, varieties of technologies have been proposed, (refer to JP-B 4-63729 and JP-A 10-216499).
When a granulator of the above spout-fluid bed type is applied for granulating a urea aqueous solution, granulation, drying and cooling simultaneously proceed in the granulator. During the above operation, it is known that smaller droplets of aqueous solution being sprayed against the particles as nuclei give larger drying effect.
With a single-fluid nozzle which deals only with liquid, an applicable method for decreasing the droplet size may be the one to increase the spraying pressure of the aqueous solution, and the one to decrease the throughput per single nozzle from which the aqueous solution is sprayed. To increase the pressure for spraying the aqueous solution, however, the apparatus for increasing the pressure becomes large. In addition, if the throughput per single nozzle is decreased, the number of nozzles arranged in the granulator has to be increased, which increases the size of the granulator to accept these nozzles, and increases the air rate to form the spout-fluid bed, thereby increasing the operating cost.
According to the design of the related art, generally the design of nozzle is conducted under the condition of the spraying pressure of aqueous solution in a range from 0.6 to 1.2 MPa and the throughput per single nozzle in a range from 0.6 to 2.0 ton/hr to achieve the optimum design as the granulator. As a result, a nozzle according to the related art has a limitation of the droplet size of 300 μm at the minimum.
According to JP-B-56-47181, there is proposed the adoption of an atomizing nozzle which accepts high pressure air for spraying the aqueous solution to mix the air with the aqueous solution before spraying the aqueous solution, thus forming very fine aqueous solution particles to conduct granulation with an improved drying effect. Since, however, the method needs to increase the air pressure, generally up to 1 to 2 bar, the method has a problem of increase in the power for pressurizing the air, which increases the operating cost.